1. Field of the Invention
The present invention relates to a laser drive circuit for controlling a laser beam such as a magneto-optical disk drive, and more particularly to a laser drive circuit for precisely controlling a laser beam intensity.
2. Related Background Art
In a prior art magneto-optical disk drive, a laser beam is irradiated to a recording medium having a magnetic film having a vertical magnetic anisotropy to locally raise a temperature while an external magnetic field is applied to orient the magnetization at the local point along a direction of the magnetic field.
In the magneto-optical recording, since the light beam and the magnetic field are used, a magnetic field modulation system in which a signal is converted to a magnetic field for recording, or a light modulation system in which the signal is converted to the turn-on and the turn-off of a laser beam for recording may be used. In the light modulation system, a magnetic field is applied along a direction of record and the signal is recorded by turning on and off the laser beam.
In the light modulation system, since the laser beam is turned on and off, a laser drive circuit for modulating a laser beam power between a maximum power and a minimum power in accordance with the values (binary values) of the record signal is required.
FIG. 1 shows a configuration of a prior art laser drive circuit disclosed in Japanese Laid-Open Patent Application No. 2-166636. A portion of a laser beam L1 emitted from a semiconductor laser LD is detected by a photo-detector PD. The detection output is amplified by an amplifier 1 which produces a detection signal S.sub.PDIO, which is applied to two sample-and-hold circuits 2 and 3 which sample and hold the detection signal. A timing of the sampling and the holding is controlled by a pulse generator 4 which receives a record signal S.sub.REC and generates a sampling pulse S.sub.PH at the rise of the record signal S.sub.REC and generates a sampling pulse S.sub.PL at the fall of the record signal S.sub.REC. The timing of the sampling and the holding is determined in accordance with the condition of the record signal. When the record signal S.sub.REC is at an H-level, a laser beam intensity from the semiconductor laser LD is maximum, and when it is at an L-level, the laser beam intensity from the semiconductor laser LD is minimum. The control to modulate the laser beam intensity in this manner is conducted by a laser drive amplifier 5.
When the record signal S.sub.REC is at the H-level, the detection signal S.sub.PDIO is sampled and held by the sample and hold circuit 2, and when the record signal S.sub.REC is at the L-level, the detection signal S.sub.PDIO is sampled and held by the sample and hold circuit 3. The timing of the sampling and the holding is controlled by the pulse generator 4, which receives the record signal S.sub.REC and generates the sampling pulse S.sub.PH at the rise of the record signal S.sub.REC and generates the sampling pulse S.sub.PL at the fall of the record signal S.sub.REC.
The hold voltages V.sub.HH and V.sub.HL produced by sampling and holding by the two sample and hold circuits 2 and 3 are applied to differential amplifiers 6 and 7, respectively. The differential amplifier 6 produces a maximum error voltage V.sub.ERH which is a difference between the hold voltage V.sub.HH and a maximum reference voltage V.sub.RH, and the differential amplifier 7 produces a minimum error voltage V.sub.ERL which is a difference between the hold voltage V.sub.HL and a minimum reference voltage V.sub.RL. The maximum error voltage V.sub.ERH and the minimum error voltage V.sub.ERL are applied to the laser drive amplifier 5 so that a maximum power and a minimum power of the laser beam L1 of the semiconductor laser LD are feedback-controlled.
FIG. 2 shows a configuration of the laser drive amplifier 5 disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 2-166636. In the laser drive amplifier 5, the record signal S.sub.REC and an inverted record signal are applied to first and second input terminals a and b, and the maximum error voltage V.sub.ERH and the minimum error voltage V.sub.ERL are applied to third and fourth input terminals c and d.
The laser drive amplifier 5 controls the laser drive current Id in accordance with the level of the input record signal S.sub.REC so that the laser beam at the minimum power or the maximum power is emitted from the semiconductor laser LD.
In this manner, the laser beam emitted from the semiconductor laser is modulated between the maximum power and the minimum power, and the maximum power and the minimum power are separately feedbackcontrolled.
By the above configuration, the laser beam intensity is controlled to the predetermined maximum power or minimum power.
The prior art laser drive circuit of the magneto-optical disk drive has the following disadvantages.
An operational characteristic of the sample and hold circuit in the prior art laser drive circuit generally includes a linearity error and a gain error.
The linearity error and the gain error are further explained. An input voltage and a hold voltage of the sample and hold circuit have a relationship as shown in FIG. 3. A characteristic between the input voltage and the hold voltage generally includes the linearity error and the gain error.
Theoretically, the input voltage and the hold voltage are in a proportional relationship, but in actual a graph showing the relationship therebetween is not linear as shown in FIG. 3 in which the increase of the hold voltage decreases as the input voltage becomes higher. An error caused by this characteristic is called the linearity error. Thus, the linearity error is larger as the input voltage becomes higher.
Theoretically, a gain of the hold voltage to the input voltage is unity (that is, a graph of the input voltage versus the hold voltage is a straight line with a unity gradient), but in actual it is smaller than unity. An error caused by this characteristic is called the gain error.
The hold voltage produced by the sample and hold circuit includes the errors due to the linearity error and the gain error. When such a hold voltage is fed back to the semiconductor laser to control the laser beam intensity, it is not possible to exactly control the laser beam intensity to the predetermined level.
When a sample and hold circuit with a small error is to be designed, a construction is complex and a cost is high.
Further, it is required that the modulation of the laser beam power between the maximum power and the minimum power is done at a high speed. This requirement is further emphasized when a higher recording density of the optical recording medium is desired. In order to modulate the laser beam at the high speed, it is necessary that the sample and hold circuit operates at the high speed, but the above error is larger in a high speed sample and hold circuit. To reduce the error, the cost is higher.
The above problems occur in the record mode as well as in the reproduction and erase modes. In the prior art laser drive circuit, since the detection signal from the photo-detector is applied to the sample and hold circuit even in the reproduction and erase modes, it is affected by the linearity error and the gain error so that the laser beam intensity cannot be exactly controlled to the predetermined level.
In the prior art laser drive circuit, when the minimum reference voltage is varied in the differential amplifier 6 to vary the minimum error voltage applied to the input terminal d (see FIG. 3) of the laser drive amplifier 7, a current I1 which flows in a transistor Q4 also varies. As a result, when a transistor Q3 is activated, the maximum power of the laser beam varies with the change of the current. In order to prevent the change of the maximum power, the maximum error voltage must be set in accordance with the change of the minimum error voltage, and the control is complex.